Light-sensitive emulsions used in the photographic art are typically prepared from a discrete phase, known as grains, containing a light-sensitive component such as silver bromide and a continuous phase containing a peptizer and a binder. The peptizer is introduced into the emulsion during precipitation of the light sensitive component to prevent coalescence or flocculation. Preferred peptizers are gelatin, i.e., alkali-treated gelatin (cattle bone or hide gelatin) or acid-treated gelatin (pigskin gelatin), and gelatin derivatives, i.e., acetylated gelatin or phthalated gelatin.
The reproducible manufacture of gelatin is important because the characteristics of the gelatin are important to the photographic emulsion properties, in particular, the light-sensitivity of the emulsion, grain morphology, and latent image stability. It has been recognized as beneficial in the manufacture of gelatin to employ oxidizing treatments in order to destroy microorganisms and other harmful impurities. The use of oxidants such as hydrogen peroxide was suggested by Sheppard (S. E. Sheppard, Great Britain Patent No. 245,456) for the elimination of various impurities in the gelatin, and is similarly mentioned more recently by F. J. Moll, J. Photog. Sci. 37, 14 (1989).
Methionine is a component of gelatin and the control of its content is an important factor in the reproducible manufacture of gelatin. However, methionine oxidizes readily to methionine sulfoxide in the presence of oxidants such as hydrogen peroxide, sodium peroxide and hypochlorite as determined by Pouradier and Rondeau (J. Photog. Sci. 16, 68 (1968)). The methionine content of the gelatin is thereby altered by the prescribed oxidation treatment.
Maskasky in U.S. Pat. No. 4,713,320 demonstrated that oxidation of the methionine (thereby reducing methionine content) in gelatin used in a tabular grain emulsion will dramatically alter the grain morphology, thickness and rod content. Fox and Holden in U.S. Pat. No. 4,713,320 describe photographic gelatins with enhanced levels of methionine. The methionine level is an important parameter in the preparation of photographic grade gelatin. It is therefore desirable for a manufacturer to be able to control the methionine content of the gelatin.